Human herpesvirus 8 (HHV-8) specifies four viral interferon regulatory factor homologues (vIRFs 1-4) that function to inhibit cellular IRFs in addition to other components of cellular defense pathways that promote cell cycle arrest and apoptosis in response to virus infection. Cellular proteins targeted for inhibition by vIRF-1 include p53, ATM, GRIM19, Smad transcription factors, and p300/CBP transcriptional co-activators required for IRF-mediated responses. We have identified an entirely novel class of interaction, between vIRF-1 and stress-responsive, pro-apoptotic BH3-only proteins (BOPs) Bim and Bid. Interactions of vIRF-1 with Bim and Bid occur via residues 170-187 (BOP-binding domain, BBD) of the viral protein and the functional BH3 domains of the BOPs. Both Bim and Bid are induced during HHV-8 productive replication, each is inhibited functionally by vIRF-1 association, and we have identified partial localization of vIRF-1 to mitochondria, consistent with the hypothesis that direct targeting and inactivation of BOPs at their site of action is biologicaly important. For Bim, a demonstrated powerful negative regulator of HHV-8 replication, we have also shown that vIRF-1 binding leads to nuclear translocation, representing a secondary mode of inactivation. These properties of vIRF-1 represent new paradigms of viral control of host responses to infection. This application is focused on examining: (1) the structural requirements of vIRF-1 mitochondrial localization and associated activities; (2) the molecular basis of BOP/BH3 targeting by vIRF-1; (3) the functional significance of individual vIRF-1:BOP interactions and of vIRF-1 mitochondrial localization in HHV-8 biology. The project will characterize unique properties and activities of vIRF-1, thereby expand understanding of viral evasion from host cell defenses, and potentially enabling future development of novel antiviral agents.